Congenital heart defects (CHDs) are a group of common (1% of live births) and serious birth defects. These conditions are the leading cause of infant mortality and multiple heart surgeries are often required for surviving cases, with typical medical costs exceeding $200,000 per case. Despite the significant impact of CHDs at both the individual- and population-level, the etiology is unknown in most cases and may differ between CHD subtypes (e.g. conotruncal versus left ventricular outflow tract defects). Several factors related to pesticide exposure have been associated with risk for CHDs. We will assess one pesticide that is a potent endocrine disruptor and a common tap water contaminant in the U.S and is suspected to increase CHD risk based on animal and epidemiologic studies. However, although tap water is the most likely source of exposure to this pesticide in the general population, no study has specifically assessed the association between exposure in tap water and CHDs in humans. With U.S. policymakers re-evaluating the safety of pesticide use, there is an urgent need to assess the potential teratogenicity of this pesticide for CHDs. We will evaluate the relationship between pesticide exposure through home tap water and CHDs using existing data from two of the largest population-based studies of birth defects: the Texas Birth Defects Registry (N=24,291 cases delivered 1999-2008) and the National Birth Defects Prevention Study (NBDPS, N=4,018 cases delivered 2000-2005). Further, we will assess the effect of exposure after accounting for the potential effects of tap water exposure to 19 additional common pesticides. Exposure will be based on measured values for the public water supply district facility of each subject during early pregnancy, and on extensive self-reported water use and consumption data for all NBDPS subjects. This study could lead to the identification of modifiable CHD risk factors. For example, if associations are confirmed, CHD risk could be reduced by implementing residential or water system interventions to reduce exposure to these contaminants in tap water.